Method of manufacturing foundry sand molds

ABSTRACT

Mixing (a) a carboxylic acid, (b) a divalent or trivalent metal oxide or hydroxide and (c) a foundry molding sand in the presence of an aqueous solution of (d) a carboxyl-group-containing polymer or (e) water followed by shaping and curing can provide sand molds having sufficiently high green strength without contamination of the working environment. The molding sand composition has excellent processability, which means filling of shapes or molds with the sand can be done with ease. The sand molds manufactured in this way can be disintegrated by a simple procedure. Therefore the molding sand can be used over and over again and does not cause environmental pollution due to disposal of the molding sand.

This is a divisional of application Ser. No. 105,534, filed Dec. 20,1979 and now U.S. Pat. No. 4,330,459.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of manufacturing foundry sand molds(including cores). More particularly, it relates to an improved methodof manufacturing sand molds which comprises mixing (a) a carboxylicacid, (b) a divalent of trivalent metal oxide or hydroxide and (c) afoundry molding sand in the presence of an aqueous solution of (d) acarboxyl-group-containing polymer of water (e) followed by shaping andhardening.

2. Description of the Prior Art

Foundry sand molds have been manufactured by adding a variety ofinorganic or organic binders to a foundry sand. However, each of theseinorganic and organic binders has its own merits and demerits. Thus theyare not always satisfactory. When water glass, for instance, is used asa binder, the molding sand can be used only once and disposal of theused sand may result in gradual dissolution of the water glass adheringto the sand surface, which means leaching of alkaline substances andconsequent alkalinization of the surrounding soil, leading not only towithering of trees and plants but also to killing of organisms living inthe soil and in the water. When organic binders such as self-curingfuran or phenol resins are used, the organic materials can be removedafter use by burning the sand and the sand can be reused. However, inburning the sand, a bad smell is produced. Moreover, the rate of curingof the binder and the mechanical strengths of the sand mold varysignificantly depending on such factors as properties of the sand,moisture content thereof, temperature, humidity, kind of acid used asbinder and concentration thereof and consequently handling of themolding sand is a task of considerable difficulty. It is also adisadvantage that such a binder must be used up within several monthsafter preparation because polymerization of the binder proceedsgradually during storage thereof.

According to the method disclosed in Japanese Patent Publication No.6259/1953 or U.S. Pat. No. 2,765,507, which uses an acrylic acid polymeras a binder, or according to the method disclosed in Japanese PatentPublication No. 8708/1961 (Patent of Addition to the above), which usesan acrylic acid-acrylonitrile copolymer as a binder, no unpleasant smellis produced in the manufacture of sand molds or in the pouring of moltenmetal, but green strengths are very low, whereby roasting is requiredfor practical use. In the present time when extra-large castings aremanufactured, it is difficult to prepare an apparatus for such roastingand therefore such binders cannot be employed in foundry practice.Although it is known that green strength can be increased by adding suchmetal oxides and hydroxides as magnesium oxide and aluminum hydroxide ascuring agents for the binders used according to the above-mentionedmethod, mere standing of the sand mold in atmospheric conditions withoutroasting thereof after manufacture can indeed lead to easy hardening ofthe surface portion but the inner part does not harden so that only verylow green strength is obtained. The resulting sand mold therefore cannotendure the load to be carried thereby in pouring molten metal in themanufacture of relatively large castings. In addition, where there is noexposure to the atmosphere, moisture originating from the aqueoussolution of the polymer used as a binder remains in the sand mold. Onpouring molten metal, the moisture in the molding sand, i.e. the mold,is converted into water vapor by the high temperature of the moltenmetal. The vapor blows out and causes formation of uneven surfaces ofthe casting. It is also a disadvantage that castings having smoothsurfaces cannot be obtained.

Another method of manufacturing foundry sand molds is disclosed inJapanese Patent Application Laid-Open No. 129,121/1978 or German PatentApplication Laid-Open (Offenlegungesschrift) No. 2,815,753. The methodcomprises adding polyacrylic acid, aluminosilicate glass, tartaric acidand/or molten granular magnesia to a molding sand followed by mixing inthe presence of water and shaping. However, this method has somedisadvantages. Thus, since the principal component of the curing agentis the aluminosilicate glass, crushing after casting is troublesome andaccumulation of the glass caused by repeated use may make further reusea hard task. Furthermore, the moisture content in the sand mold asmanufactured amounts to 2 to 5%, which is far above the preferablemoisture content of not more than about 1%.

Furthermore, the method to make foundry sand mold by kneading thefoundry sand adding citric acid, metal oxide and water is known in theJapanese Patent Application No. 27404/1976. This method has thedisadvantages of shortening the working time, becoming rapidly stickywhen an ordinary metal oxide is used, and becoming incapable ofpractical use because of the remarkably low residual green strength athigh temperature. In addition, a method using a combination of metaloxides such as zinc oxide, hydroxylic acids such as tartaric acid andwater as binder components is also disclosed in the Japanese PatentApplication Laid-Open No. 126326. However, this method has the samedisadvantages as mentioned above.

SUMMARY OF THE INVENTION

This invention is directed to a solution of the abovementioned problemsfound in the prior art. Accordingly, the first object of the inventionis to provide a method of manufacturing foundry sand molds which can beused in practice without forced drying and have sufficient greenstrength throughout the mold.

The second object is to provide manufacturing foundry sand molds havingsmooth surfaces.

The third object is to provide a method of manufacturing foundry sandmolds which can easily be disintegrated simply by spraying used sandmolds with small amounts of water and also a method of manufacturingfoundry sand molds which makes it possible to use the molding sand sodisintegrated over and over again.

The fourth object is to provide a method of manufacturing foundry sandmolds which does not produce any unpleasant smell or any toxic gasesduring the manufacture of the sand molds and eliminate the possibilityof environmental pollution such as contamination of the soil withalkalies by disposal of used molding sands.

The fifth object is to provide a method of manufacturing foundry sandmolds which have at high temperature residual strength large enough forpractical use.

The sixth object is to provide a method of manufacturing foundry sandmolds which is excellent in workability and gives molding sandcompositions that can be prepared by easy working processes of mixingingredients, said composition showing no significant increase inviscosity over at least 30 minutes after mixing the components, it beingpossible to fill shapes with the composition in a simple manner withincreased specific gravities of the fillings being attainable.

According to the present invention, the first to fifth objects describedabove are achieved by mixing (a) at least one carboxylic acid selectedfrom the group consisting of polycarboxylic acids each having two ormore carboxyl groups per molecules and hydroxylcarboxylic acids eachhaving at least one hydroxyl group and at least one carboxyl group permolecule, (b) a divalent or trivalent metal oxide or hydroxide and (c) afoundry molding sand in the presence of an aqueous solution of (d)carboxyl-group-containing polymer followed by shaping and curing. Whenespecially higher green strength is required, such strength is obtainedmainly with the combination of maleic anhydride, malic acid or citricacid as carboxylic acid (a) and magnesium oxide or magnesium hydride asdivalent or trivalent metal oxide or hydroxide. When especially higherresidual strength at high temperature is required, such strength isobtained mainly by using an aqueous solution of (d)carboxyl-group-containing polymer, preferably polyacrylic acid.

Furthermore, the first to fourth and sixth objects are achieved bymixing (a) at least one carboxylic acid selected from the groupconsisting of polycarboxylic acids each having two or more carboxylgroups per molecule and hydroxycarboxylic acids each having at least onehydroxyl group and at least one carboxyl group per molecule, (b')magnesium oxide having a size of 220 to 320 angstroms in the directionof the [200] direction of the crystal thereof as measured by X-raydiffractometry as the divalent or trivalent metal oxide and (c) afoundry molding sand in the presence of (e) water in place of an aqueoussolution of (d) carboxyl-group-containing polymer (wherein residualstrength at high temperature is not necessary to some extent) followedby shaping and curing.

Accordingly, the best mode of practicing the present invention consistsin the use of citric acid, maleic anhydride or malic acid as thecarboxylic acid (a), magensium oxide having a size of 220 to 320angstroms in the direction of the [200] direction of the crystal thereofas measured by X-ray diffractometry as the divalent or trivalent metaloxide or hydroxide (b) and an aqueous solution of polyacrylic acid as anaqueous solution of carboxyl-group-containing polymer (d).

DETAILED DESCRIPTION OF THE INVENTION

The foundry molding sand to be used in practicing the invention may beany of conventional molding sands. The sand compositions according tothe invention have the merit that they can be reused. While the sandmolds after casting are already subject to disintegration, the additionof water makes disintegration occur even more easily. Therefore, themolding sand is scarcely damaged or broken and, when fresh binder isadded to cover up the loss caused during casting, can be used over andover again. In such repeated use, there is no need of adding thedivalent or trivalent metal oxide or hydroxide. Consequently no specificcomponents accumulate in the molding sand, so that the sand can be useda very large number of times.

The carboxylic acid (a) to be used in practicing the invention includespolycarboxylic acids such as dicarboxylic and tricarboxylic acids andhydroxycarboxylic acids. The term "polycarboxylic acids" as used hereinmeans organic compounds having 2 to 20, preferably 3 to 10, carbon atomsand containing two or more carboxyl groups or groups capable of beingconverted into carboxyl groups per molecule. Examples of suchpolycarboxylic acids are oxalic, malonic, succinic, glutaric, adipic,pimelic, suberic, azelaic and sebacic acid and other saturated aliphaticdicarboxylic acids; tricarballylic and aconitic acid and other saturatedtricarboxylic or polycarboxylic acids; maleic acid, maleic anhydride,fumaric, citraconic, mesaconic and itaconic acid and other unsaturatedaliphatic dicarboxylic acids; 2-carboxymaleic and 2-carboxyglutaric acidand other unsaturated aliphatic tri- and polycarboxylic acids; phthalic,isophthalic and terephthalic acid, phthalic anhydride and other aromaticdicarboxylic acids; trimellitic acid, trimellitic anhydride,pyromellitic acid, 1,2,4,5-benzenetetracarboxylic dianhydride,benzenepentacarboxylic acid and other aromatic tri- and polycarboxylicacids.

The term "hydroxycarboxylic acids" as used herein means organiccompounds having 2 to 20, preferably 3 to 11, carbon atoms andcontaining at least one hydroxyl group per molecule as well as at leastone carboxyl group or group capable of being converted into a carboxylgroup. Examples of such hydroxycarboxylic acids are glycolic, lactic,malic, trihydroxyglutaric, tartaric, citric and hydroxyacrylic acid andother aliphatic hydroxycarboxylic acids; hydroxybenzoic,2-hydroxyisophthalic, 4,5-dihydroxyphthalic and2,5-dihydroxyterephthalic acid and other aromatic hydroxycarboxylicacids.

To sum up, the carboxylic acids to be used according to the presentinvention are those organic compounds that have 3 to 20 carbon atoms andeither contain at least two carboxyl groups or contain at least onecarboxyl group and at least one hydroxyl group.

Among these preferred are such polycarboxylic acids as maleic anhydride,adipic acid, trimellitic acid and trimellitic anhydride and suchhydroxycarboxylic acids as citric acid and 2-hydroxyisophthalic acidbecause they develop very high green strength. Especially preferred aremaleic anhydride, malic acid and citric acid. When the green strength isrequired to be ten and odd kg/cm², citric acid should be used. Thesecarboxylic acids may be used alone or in combination of two or more ofthem.

The divalent or trivalent metal oxide or hydroxide, which is anotherimportant component to be used in practicing the invention, includesoxides and hydroxides of magnesium, aluminum, calcium, zinc, iron etc.,such as magnesium oxide, calcium oxide, zinc oxide, aluminum oxide,magnesium hydroxide, calcium hydroxide, aluminum hydroxide and ironoxide. Among these, magnesium oxide and magnesium hydroxide arepreferred. These metal oxides and hydroxides may be used either alone orin combination of two or more. Prolongation of the working time of themolding sand composition, which is one of improvements attainable by thepresent invention, can be achieved especially when specific grades ofmagnesium oxide that have a size of 220 to 320 angstroms in the [200]direction of the crystal thereof as measured by X-ray diffractometry areused as the divalent or trivalent metal oxide or hydroxide (b). Suchgrades of magnesium oxide that have a size (D) in the [200] direction ofthe crystal thereof as measured by X-ray diffractometry of 220 to 320,preferably 240 to 300 angstroms can be prepared, for example, by firingin an electric furnace at temperatures of 860° to 1170° C. acommercially available light burned magnesium which has a size of 95 to210 angstroms in the [200] direction of the crystal thereof. If thecrystal size (D) is less than 220 angstroms, the working time will betoo short, and if, conversely, it exceeds 320 angstroms, sand moldstrength can scarcely be developed even after seven days of standing,hence the objects of the invention can not be achieved.

The size (D) in the [200] direction of the crystal as mentioned hereinis the value determined by the Scherrer's equation concerning X-raydiffraction.

Thus specified magnesium oxide gives not only an advantage to prolongthe forementioned working time, but also a merit to give the foundrymold having enough green strength simply by applying water and notalways by applying carboxyl-group-containing polymers.

The carboxyl-group-containing polymer to be used in practicing theinvention includes polymers having carboxyl groups and polymers havingfunctional groups which can react with water or an alkaline compound toyield carboxylic acids or carboxylic acid salts, as shown below:

(1) Homopolymers of acrylic, methacrylic, crotonic, itaconic, maleic,citraconic and mesaconic acid and monoesters thereof and other monomerscontaining one or more carboxyl groups and binary or multicomponentcopolymers of such monomers and unsaturated monomers copolymerizabletherewith such as styrene, styrene derivatives, alkenes, cyclohexene,vinyl halides, vinyl esters, allyl esters, acrylamide, acrylonitrile,methacrylonitrile, vinyl ethers, allyl alkyl ethers, acrylic acidesters, methacrylic acid esters, crotonic acid esters, amleic aciddiesters and fumaric acid diesters;

(2) Hydrolysis or alkali saponification products from homopolymers ofmonomers containing at least one functional group capable of beingconverted into a carboxyl group by hydrolysis or alkali saponificationsuch as maleic anhydride, citraconic anhydride, itaconic anhydride,acrylamide, acrylonitrile, methacrylonitrile, acrylic acid esters,maleic acid diesters and fumaric acid diesters and from binary ormulticomponent copolymers of such monomers and other monomerscopolymerizable therewith such as styrene, styrene derivatives, alkenes,cyclohexene, vinyl halides, vinyl esters, vinyl ethers and allyl alkylethers; and

(3) Polymeric materials produced by introducing carboxyl groups intomacromolecules having reactive functional groups in side chains thereofby esterification, etherification, acetalization or some other reaction,such as polyvinyl alcohol esterified with oxalic acid, polyvinyl alcoholetherified with glycolic acid, polyvinyl alcohol acetalized withglyoxalic acid and carboxymethylcellulose.

Among these carboxyl-group-containing polymers, preferred are thosementioned under (1) and (2), more specifically, polyacrylic acid,polymethacrylic acid, other polyacrylic polymers and copolymers,copolymers of alpha-olefins and maleic anhydride, and copolymers ofmaleic anhydride and styrene, vinyl acetate or methyl vinyl ether. Morepreferred are polyacrylic acid, ethylene-maleic anhydride copolymer(preferably with a maleic anhydride content of not less than 50 molepercent), isobutylene-maleic anhydride copolymer, styrene-maleicanhydride copolymer and other maleic anhydride copolymers. Especiallypreferred is polyacrylic acid. These carboxyl-group-containing polymersmay be used either alone or in combination of two or more. Thosecarboxyl-group-containing polymers that are easily soluble in water canbe used in the form of aqueous solutions using water alone as thesolvent, while those that are hardly soluble in water can be used in theform of aqueous solutions containing basic substances such as ammonia,sodium hydroxide and potassium hydroxide.

In addition to the above-mentioned essential components, there may beadded in some cases such materials as wood flour, sawdust, fire clays,silica powder, bentonite, boric acid, sulfuric acid, plastics, latexesand emulsions.

The proportion of the carboxylic acid (a) to the molding sand (c), i.e.the ratio a/c, is required to be not less than 0.0015, preferably notless than 0.005, by weight. Insufficient amounts of the carboxylic acidcannot produce the effect of addition thereof. On the other hand,excessive amounts are uneconomical. Therefore, the ratio a/c shouldpreferably be not more than 0.2, more preferably not more than 0.09.

The proportion of the divalent or trivalent metal oxide or hydroxide (b)to the molding sand (c), i.e. the ratio b/c, is required to be0.001-0.05, preferably 0.005-0.02. When an aqueous solution of acarboxyl-group-containing polymer is not used but water alone is used,the ratio b/c is 0.0025-0.03, preferably 0.01-0.02. Therefore,generally, the ratio is preferably in the range of 0.005-0.02.

The proportion of the carboxyl-group-containing polymer (d) to themolding sand (c), i.e. the ratio d/c, is desirably not more than 0.055,preferably in the range of 0.0006 to 0.055. When the amount of thecarboxyl-group-containing polymer is below this range, satisfactoryeffects of addition thereof cannot be produced and a significantincrease in green strength cannot be attained. Conversely, excessiveamounts are uneconomical. It is preferable for producing better effectsfrom this point of view that the aqueous solution of thecarboxyl-group-containing polymer should contain 20 to 95% by weight ofwater (the corresponding concentration of the carboxyl-group-containingpolymer in the aqueous solution being 5 to 80% by weight), preferablynot more than 80% by weight of water (the corresponding concentration ofthe carboxyl-group-containing polymer in the aqueous solution being notless than 20% by weight).

Water may be added in place of the aqueous solution of thecarboxyl-group-containing polymer, as the case may be. In such case, theproportion of water (e) to molding sand (c), i.e. the ratio e/c, is0.005-0.05 by weight.

The sand molds can be manufactured by adding to and mixing with moldingsand (c) carboxylic acid component (a) such as a polycarboxylic acid, amixture thereof, a hydroxycarboxylic acid, a mixture thereof or amixture of a polycarboxylic acid and a hydroxycarboxylic acid and wateror an aqueous solution of carboxyl-group-containing polymer (d) and thenadding to and mixing with the mixture divalent or trivalent metal oxideor hydroxide (b) followed by shaping in a conventional manner. The orderof addition of carboxylic acid component (a), water (e) or the aqueoussolution of carboxyl-group-containing polymer (d) and magnesium oxide tothe molding sand is not critical and always almost the same results areobtained. Although sufficient green strength can be developed bystanding at ambient temperatures for several hours, the molds may beheated during standing.

The sand molds manufactured by the method of the invention are excellentin filling efficiency and therefore it is not always necessary to applypressure. However, in cases where extremely high strength is desired, anappropriate pressure may be applied in shaping the molds.

The invention will be illustrated more in detail by the followingexamples, which, however, should by no means be construed as limitingthe invention. The divalent or trivalent metal oxide or hydroxide isabbreviated to "metal compound (b)" and the carboxyl-group-containingpolymer to "polymer (d)".

I EXAMPLES OF DICARBOXYLIC ACID (A)-METAL COMPOUND (B)-MOLDING SAND(C)-AQUEOUS SOLUTION OF POLYMER (D) SYSTEMS Example 1

Maleic anhydride powder (MA) and magnesium oxide (MgO) were added to andmixed with quartz sand in the proportions shown in Table 1, then anaqueous solution of polyacrylic acid (hereinafter abbreviated to PAA)having a concentration of 40% was added. After kneading the mixture, atest specimen was prepared therefrom by filling a mold with the mixtureaccording to the method of preparing test specimens as specified inJapanese Industrial Standards (JIS) Z 2604. The mold for shaping thespecimen was allowed to stand at ordinary temperatures under isolationfrom the atmosphere for 4 hours. Thereafter, green strength wasmeasured. The amounts of quartz sand, aqueous solution of PAA, MA andMgO and the green strength after 4 hours of standing at ordinarytemperatures are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Composition                                                                            a   b*  c*  d   e*  f*  g*  h*                                       __________________________________________________________________________    Quartz sand (g)                                                                        150 150 150 150 150 150 150 150 150                                  Aqueous solution                                                              of PAA (g)                                                                             0.5 0.5 0.5 0.1 0.25                                                                              2.0 6.0 19.0                                                                              25.0                                 MA (g)   0   1.0 0.5 1.0 1.0 10.0                                                                              2.0 2.0 2.0                                  MgO (g)  0.5 0.5 0.5 0.2 0.2 1.5 1.5 3.0 3.0                                  Green strength                                                                (kg/cm.sup.2)                                                                          0.1 1.81                                                                              1.68                                                                              0   0.86                                                                              3.16                                                                              3.31                                                                              5.35                                                                              0.24                                 __________________________________________________________________________     *Compositions falling under the scope of the present invention.          

As can be seen in Table 1, green strength is too low for practicalpurposes with composition (a) lacking maleic anhydride. Even when thecomposition contains all the constituents specified by the presentinvention, sufficient green strength cannot be obtained if the amountsof the constituents are not within the range specified by the presentinvention (cf. compositions d and i). On the other hand, foundry sandmolds having green strength sufficient for practical use can bemanufactured with compositions b, c, e, f, g and h which are provided bythe present invention.

Example 2

An aqueous solution of polyacrylic acid (PAA) having a concentration of40% and maleic anhydride powder (MA) were added to and mixed with quartzsand, then magnesium oxide powder (MgO) (Kyowamag 20,manufactured byKyowa Gas Kagaku Kogyo K.K.) was added and the resulting mixture waskneaded and shaped into a test specimen by filling a mold with themixture by the method of preparing test specimens as specified in JIS Z2604. The specimen was allowed to stand at ordinary temperatures for 4hours under isolation from the atmosphere. Green strength was thenmeasured. The results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Composition 2-a    2-b*   2-c* 2-d* 2-e* 2-f* 2-g                             ______________________________________                                        Quartz sand (g)                                                                           150    150    150  150  150  150  150                             Aqueous solution                                                              of PAA (g)  2.0    7.5    2.0  5.5  7.5  7.5  7.5                             MA (g)      2.0    2.0    2.0  2.0  2.0  2.0  2.0                             MgO (g)     0.05   0.25   1.0  1.5  3.0  6.0  9.0                             Green strength                                                                (kg/cm.sup.2)                                                                             0      0.82   3.87 3.75 5.75 0.87 0                               ______________________________________                                         *Compositions covered by the present invention.                          

As can be seen in Table 2, sufficient green strength could not beobtained when the proportion of MgO to the molding sand was 0.0003(=0.05/150) by weight. On the contrary, sufficient green strength couldbe obtained when the proportion was 0.00166 (=0.25/150) to 0.04(=6.0/150).

Example 3

Maleic anhydride powder (MA) (2 g) and 1.5 g of a metal oxide orhydroxide specified in Table 3 were mixed with 150 g of quartz sand andthen 7 g of an aqueous solution of polyacrylic acid (PAA) having aconcentration of 40% was further added. The resulting mixture waskneaded and shaped into a test specimen by filling a mold therewith bythe method of preparing test specimens as specified in JIS Z 2604. Thetest specimen was allowed to stand at ordinary temperatures for 4 hoursunder isolation from the atmosphere. Green strength was then measured.The results are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Kind of    Calcium  Zinc    Calcium Magnesium                                 metal oxide                                                                              oxide    oxide   hydroxide                                                                             oxide                                     or hydroxide                                                                             CaO      ZnO     Ca(OH).sub.2                                                                          MgO                                       ______________________________________                                        Green strength                                                                (Kg/cm.sup.2)                                                                            3.75     3.27    1.25    6.81                                      ______________________________________                                    

Example 4

Two grams (2 g) of maleic anhydride powder (MA) and 1.5 g of magnesiumoxide (MgO, Kyowamag 20) were mixed with 150 g of quartz sand, and 3 gof an aqueous solution of polyacrylic acid (PAA) having a concentrationof 40% was further added. The mixture was kneaded and a test specimenwas prepared therefrom by filling a mold therewith according to JIS Z2604. The specimen was allowed to stand for 4 hours under isolation fromthe atmosphere and then green strength was measured. Separately, a testspecimen was prepared by the same procedure as above and placed on aniron plate heated at 1,000° C. and kept there for 5 minutes. Thespecimen was removed from the iron plate, crushed and, after stirringfor homogenization, analyzed chemically. The losses of PAA, MA and waterwere 0.435 g, 0.725 g and 1.65 g, respectively. To the sand aftercrushing, there were added 0.725 g of MA, 1.09 g of the 40% PAA solutionand 1.0 g of water. The resulting mixture was kneaded and shaped into atest specimen by filling a mold therewith by the above-mentioned methodand green strength was measured in the same manner as mentioned above.This procedure was repeated four times more and each time green strengthwas measured. The results are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        No. of repetition 1      2      3    4    5                                   ______________________________________                                        Green strength (kg/cm.sup.2)                                                                 6.06   7.34   4.33 3.72 5.10 4.56                              ______________________________________                                    

As can be seen in Table 7, no significant decrease in green strength wasobserved even after five times repeated use. It could be concluded fromthe results that the method of the present invention can be used inactual practice.

II. EXAMPLES OF POLYCARBOXYLIC ACID (a)-METAL COMPOUND (b)-MOLDING SAND(c)-AQUEOUS SOLUTION OF POLYMER (d) SYSTEMS Example 5

Polycarboxylic acids (PCA) containing 3 or more carboxyl groups permolecule and an aqueous solution of polyacrylic acid (PAA) having aconcentration of 30% were added to and mixed with quartz sand in theproportions specified in Table 5 and then magnesium oxide (MgO) wasadded. Test specimens were prepared by filling a mold with each of theresulting mixtures by the method of preparing test specimens asspecified in JIS Z 2604, allowed to stand at ordinary temperatures for 4hours and tested for green strength. The amounts of quartz sand, PCA,aqueous solution of PAA and MgO and the green strength after 4 hours ofstanding at ordinary temperatures are shown in Table 5.

                                      TABLE 5                                     __________________________________________________________________________    Composition                                                                            5-a    5-b   5-c   5-d   5-e                                         __________________________________________________________________________    Quartz sand (g)                                                                        150.0  150.0 150.0 150.0 150.0                                          Kind  Tricarballylic                                                                       2-carboxy-                                                                          Trimellitic                                                                         Trimellitic                                                                         Pyromellitic                                PCA      acid   maleic acid                                                                         acid  anhydride                                                                           acid                                           Amount (g)                                                                          3.0    4.0   3.0   3.0   4.0                                         Aqueous solution                                                              of PAA (g)                                                                             2.0    2.3   2.1   2.2   2.0                                         MgO (g)  1.5    1.3   1.5   1.2   1.5                                         Green strength                                                                (kg/cm.sup.2)                                                                          3.54   2.95  5.72  5.91  4.17                                        __________________________________________________________________________

As can be seen in Table 5, sufficiently high green strength can bedeveloped by using the polycarboxylic acids having 3 or more carboxylgroups per molecule together with the aqueous solution of PAA and MgO inthe proportions specified in Table 5 followed by 4 hours of standing atordinary temperatures. On the contrary, when the amount of aqueoussolution of PAA was reduced to 0.1 g with the amounts of quartz sand,trimellitic acid and MgO being maintained respectively at the samelevels as in compositions c in Table 5, the green strength was too lowto be measured (green strength 0.0 kg/cm²). When the amount of MgO wasreduced to 0.05 with the amounts of quartz sand, trimellitic anhydrideand aqueous solution of PAA being maintained respectively at the samelevels as in compositon d in Table 5, the green strength was too low tobe measured.

Example 6

Trimellitic acid (hereinafter abbreviated to TMA) was mixed with maleicanhydride (MA) in the two proportions specified in Table 6. Eachmixture, together with an aqueous solution of polyacrylic acid (PAA)having a concentration of 30%, was added to and mixed with quartz sand,and then magnesium oxide (MgO), magnesium hydroxide (Mg(OH)₂) or ironoxide (Fe₂ O₃) was added to prepare the sand compositions shown in thetable. Test specimens were prepared by filling a mold with each of theresulting mixtures or sand compositions by the method of preparing testspecimens as specified in JIS Z 2604, allowed to stand at ordinarytemperatures for 4 hours and tested for green strength. The ratios ofTMA to MA, the amounts of TMA-MA mixture, water, aqueous solution ofPAA, MgO, Mg(OH)₂ and Fe₂ O₃ as well as the green strength valuesobtained after 4 hours of standing at ordinary temperatures are shown inTable 6.

                  TABLE 6                                                         ______________________________________                                        Composition        6-a      6-b     6-c                                       ______________________________________                                        Quartz sand (g)    150.0    150.0   150.0                                     TAM/MA ratio (g)   90/10    80/20   80/20                                     Amount of TMA--MA mixture (g)                                                                    3.0      3.0     3.0                                       Aqueous solution of PAA (g)                                                                      2.1      2.1     2.1                                       MgO (g)            1.5      --      --                                        Mg(OH).sub.2 (g)   --       1.5     --                                        Fe.sub.2 O.sub.3 (g)                                                                             --       --      3.5                                       Green strength (kg/cm.sup.2)                                                                     5.28     1.27    0.95                                      ______________________________________                                    

As can be seen in Table 6, sufficiently high green strength can beobtained after 4 hours of standing at ordinary temperatures when suchmixtures of trimellitic acid (TMA) and maleic anhydride (MA) are addedto quartz sand in combination with an aqueous solution of PAA and withany one of MgO, Mg(OH)₂ and Fe₂ O₃.

III. EXAMPLES OF HYDROCARBOXYLIC ACID (a)-METAL COMPOUND (b)-MOLDINGSAND (c)-AQUEUS SOLUTION OF POLYMER (d) SYSTEMS Example 7

Each of the hydroxycarboxylic acids (hereinafter abbreviated to OCA)shown in Table 7 and water or an aqueous solution of polyacrylic acid(PAA) having a concentration as specified in the table were added to andmixed with quartz sand and then magnesium oxide (MgO) was added. Eachmixture, after mixing up, was shaped into a test specimen by filling amold therewith by the method of preparing test specimens as specified inJIS Z 2604. The specimen was allowed to stand at ordinary temperaturefor 4 hours and tested for green strength. The results are shown inTable 7.

                  TABLE 7                                                         ______________________________________                                        Composition  7-a    7-b    7-c   7-d     7-e                                  ______________________________________                                        Quartz sand (g)                                                                            200    200    200   200     200                                  OCA kind     Lac-   Cit-   Tar-  2-Hydroxy                                                                             Malic                                             tic    ric    taric isophthalic                                                                           acid                                              acid   acid   acid  acid                                         (g)          3.0    3.5    4     4       3.5                                  Concentration of                                                              aqueous PAA                                                                   solution (%) 40     20     30    40      20                                   (g)          3.0    2.5    2.5   3.0     2.4                                  MgO (g)      1.5    2.0    2.0   2.0     1.4                                  Compression                                                                            after                                                                strength 4 hrs.  1.1    11.7 2.8   3.5     6.5                                (kg/cm.sup.2)                                                                          after                                                                         24 hrs.        17.0               15.3                               ______________________________________                                    

As can be seen in the table, adequate green strength could be developedafter 4 hours of standing at ordinary temperatures by mixing any one oflactic acid, citric acid, tartaric acid, 2-hydroxyisophthalic acid, andmalic acid together with an aqueous solution of PAA and MgO, with quartzand followed by shaping. As in the cases of compositions 7-b, the use ofcitric acid can provide sand molds having especially high strength. Whenthe amount of MgO was reduced to 0.05 g, which was smaller than thelower limit specified by the present invention, with the amount of theremaining components being maintained respectively at the same levels asin composition 7-b, the green strength was too low to be measured.

Example 8

Each of the mixtures of various hydroxycarboxylic acid (OCA) and variouspolycarboxylic acids (PCA) as shown in Table 8 and an aqueous solutionof polyacrylic acid (PAA) having a concentration as specified in Table 8were added to and mixed with quartz sand and then magnesium oxide (MgO),magnesium hydroxide (Mg(OH)₂ or iron oxide (Fe₂ O₃) was added. Aftermixing up, each of the resulting mixtures was shaped into a testspecimen by filling a mold therewith by the method of preparing testspecimen as specified in JIS Z 2604. The test specimen was allowed tostand at ordinary temperatures for 4 hours and tested for green stength.The results are shown in Table 8.

                  TABLE 8                                                         ______________________________________                                        Composition 8-a       8-b         8-c                                         ______________________________________                                        Quartz sand (g)                                                                           200       200         200                                         OCA Kind    Citric acid                                                                             Citric acid Citric acid                                 (g)         2.5       1.5         2.0                                         PCA Kind    Trimellitic                                                                             Trimellitic Tricarballylic                                          anhydride acid        acid                                        (g)         1.0       2.0         1.5                                         Concentration of                                                              aqueous solution                                                              of PAA (%)  20        40          30                                          (g)         2.5       3.0         2.5                                         MgO (g)     --        --          2.0                                         Mg(OH).sub.2 (g)                                                                          1.0       --          --                                          Fe.sub.2 O.sub.3 (g)                                                                      --        5.0         --                                          Compression                                                                   strength                                                                      (kg/cm.sup.2)                                                                             3.28      1.15        5.62                                        ______________________________________                                    

As can be seenin Table 8, adequate green strength can be obtained after4 hours of standing at ordinary temperatures when hydroxycarboxylic acidsuch as citric acid is used in admixture with polycarboxylic acids suchas trimellitic anhydride, trimellitic acid and tricarballylic acid.

IV. EXAMPLES OF CARBOXYLIC ACID (a)-SPECIFIC MAGNESIUM OXIDE (b)-MOLDINGSAND (c)-AQUEOUS SOLUTION OF POLYMER (d) SYSTEMS

The size (D) of a specific magnesium oxide in the [200] direction of thecrystal thereof as measured by X-ray diffractometry is the valuedetermined by the following procedure.

Apparatus: Geiger Flex D-3F, Rigaku Denki K.K.

Measuring conditions: 35 KV, 10 mA, Cu Ka ray made monochromatic througha nickel (Ni) filter; wavelength of the X-ray (λ)=1.5418 angstroms

Calculation of the size (D) in the [200] direction of the crystal:

The following Scherrer's equation is used: ##EQU1## where K is theScherrer's constant,

B is the half-width of the peak given by the X-ray diffracted by the[200] plane (2θ=43°) of magnesium oxide, and

b is the equipment constant.

Example 9

Quartz sand (1600 g), 20 g of a 20% aqueous solution of polyacrylic acidand 32 g of citric acid were blended and then 12 g of each ofmagnesiumoxide (MgO) species having various crystal sizes (D) in the[200] direction as shown in Table 9 was added, followed by mixing up.Test specimens were prepared by filling molds with the mixturesimmediately after the mixing and after 5, 10 and 20 minutes of standingafter the mixing by the method of preparing tests specimens as specifiedin JIS Z 2604, then allowed to stand at ordinary temperatures for 24hours and tested for green strength. The results are shown in Table 9.The magnesium oxide species having desired crystal sizes were preparedby roasting.

                  TABLE 9                                                         ______________________________________                                                       Green compression                                                     Crystal size                                                                          strength (kg/cm.sup.2) after                                          of MgO (D)                                                                            0 min.   5 min.  10 min.                                                                              20 min.                                ______________________________________                                        Comparison a                                                                           200 A     6.75     5.50  1.50   0.25                                 Example b                                                                              240       8.55     8.20  8.75   7.65                                 Example c                                                                              270       11.21    12.74 15.29  14.52                                Example d                                                                              300       3.85     3.52  3.55   3.45                                 Comparison e                                                                           360       0.15     0.10  --     --                                   ______________________________________                                    

As can be seen in Table 9, the sand compositions of Comparative Examplea made with the magnesium oxide species having crystal sizes (D) of 200and 360 angstroms, when formed into test specimens after standing for 10minutes or more following the mixing, gave significantly decreasedstrength as compared with that of the test specimens made immediatelyafter the mixing, whereas, in Examples b, c and d where the magnesiumoxide species having crystal sizes (D) of 240, 270 and 300 angstroms,respectively, were used, the strength of each test specimen made 20minutes after the mixing was not so different from that of thecorresponding specimen prepared immediately after the mixing. InComparative Example e where the magnesium oxide used had a crystal size(D) of 360 angstroms, even the test specimen prepared directly after themixing had not any adequate strength. In Comparative Example a, fillingof the mold could be done only with difficulty except for thecomposition immediately after the mixing.

For producing conditions as close as possible to those in actual castingpractice, the test specimens made in Example c were placed on an ironplate heated at 1,000° C. with the bases of the specimens brought intocontact with the iron plate, kept there for 5 minutes and allowed tocool. When the specimens were cool, they were sprayed with water. Forsome time directly after the spraying, they retained firmly their stableshapes but became more and more disintegratable with the lapse of time.One hour after the spraying with water, they could easily bedisintegrated by mere touching with the hand. The resulting sandcomposition was dried and sieved for preparation for reuse. The aqueouspolyacrylic acid solution and citric acid were added to this sand andsand molds were prepared by the above-mentioned method and tested forgreen strength. The strength values were almost the same as those shownin Table No. 9 for Example c. A number of times repetition of such reusecaused little changes in green strength.

V. EXAMPLES OF CARBOXYLIC ACID (a)-SPECIFIC MAGNESIUM OXIDE (b)-MOLDINGSAND (c)-WATER (e) SYSTEMS EXAMPLES 10,11 & 12 AND COMPARATIVE EXAMPLES1, 2 & 3

As shown in Table 10, maleic anhydride (MA), trimellitic acid (TMA) orcitric acid (CA) and water added to 200 g of quartz sand, and then twokinds of magnesium oxide (MgO) having different crystal size (D) wereeach added, followed by mixing up. Test specimens were prepared byfilling molds with mixture 0 (zero) and 20 or 25 minutes after themixing by method of preparing test specimens as specified in JIS Z 2604,allowed to stand at ordinary temperatures for 24 hours and tested forgreen strength. The results are shown in Table 10.

                  TABLE 10                                                        ______________________________________                                        Example No.         10           11         12                                Comparative No.                                                                            1             2           3                                      ______________________________________                                        Amount of quartz                                                              sand (g)     200    200    200   200   200  200                               Carboxylic acid                                                                            MA     MA     TMA   TMA   CA   CA                                Amount (g)   4.0    4.0    3.0   3.0   5.0  5.0                               Size (D) of MgO (A)                                                                        200    270    200   270   200  270                               Amount (g)   2.0    2.0    1.5   1.5   2.0  2.0                               Amount of water (g)                                                                        2.0    2.0    1.5   1.5   2.0  2.0                               Green     F.sub.0                                                                              0.25   5.75 4.81  5.10  5.04 10.5                            compression                                                                             F.sub.20                                                                             0.25   5.20                                                  strength  F.sub.25           0.1   4.55  0.20 9.7                             (kg/cm.sup.2)                                                                 F.sub.20 or F.sub.25 F.sub.0                                                                   0.05   0.91 0.02  0.89  0.04 0.92                            ______________________________________                                         F.sub.0, F.sub.20, and F.sub.25 are the green compression strength for th     specimen shaped at 0, 20, and 25 minutes after mixing.                   

As can be seen in Table 10, in Examples 10,11 and 12 where the magnesiumoxide had a crystal size (D) of 270 angstroms, the test specimens made20 or 25 minutes after the mixing revealed little difference in greenstrength as compared with those prepared directly after the mixing,whereas, in Conparative Examples 1, 2 and 3 where the magnesium oxidehad a crystal size (D) or 200 angstroms, the test specimens made 20 or25 minutes after the mixing showed remarkable decrease in green strengthas compared with those prepared immediately after the mixing.

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of the inventionas set forth herein.

What is claimed is:
 1. A method of manufacturing sand molds whichcomprises mixing (a) 0.0015-0.2 parts by weight of a polycarboxylic acidhaving 2-20 carbon atoms and containing at least two carboxyl groups permolecule or a hydroxyl carboxylic acid having 2-20 carbon atoms andcontaining at least one carboxyl group and one hydroxyl group, (b)0.001-0.05 parts by weight of a magnesium oxide having a size of 220 to320 angstroms in the [200] direction of the crystal thereof as measuredby X-ray diffractometry and (c) one part by weight of a foundry moldingsand in the presence of (e) 0.005-0.05 parts by weight of water followedby shaping and curing said shaped mixture.
 2. The method of claim 1,wherein the hydroxyl carboxylic acid is citric acid or malic acid. 3.The method of claim 1, wherein the polycarboxylic acid is maleicanhydride.